Nucleoside analogues, the derivatives of the natural nucleosides found as building blocks of DNA and RNA, are effective in the clinical treatment of human cancer or viral diseases, although in the early years such compounds were evaluated as anti-tuberculosis agents. Such compounds have been registered in the market for more than 40 years, and approximately 35 products are currently in daily use. The natural nucleosides illustrated in the FIGURE below, are constructed from two classes of nitrogen bases, the purines exemplified by adenine and guanine and the pyrimidines exemplified by thymine, uracil and cytosine, and from the monosaccharide ribose or deoxyribose.

The natural nucleosides all exist in the so called β-D configuration as illustrated in the formula A, below. The nitrogen base and the hydroxy-methyl side chain on the sugar ring are both on the same side (cis) of the plane of the sugar ring.

If the two groups are on either side (trans), it is referred to as the α isomer.
In order to obtain nucleoside derivatives with anticancer or antiviral activity, chemical modifications in either the nitrogen base and/or the monosaccharide have been performed. For instance the addition of halogen atoms, the substitution of OH groups with other functional groups or a stereochemical change from ribose to arabinose may lead to products with a potential therapeutic benefit. In many products, the monosaccharide ring is conserved, while in others, the sugar ring has been changed into a chain. The nucleoside analogues are small molecules with fair to excellent aqueous solubility.
The extensive research and development effort put into the area of nucleoside analogues due to the worldwide AIDS epidemic bolstered the basic knowledge and understanding of mechanism of action, alterations in activity profile due to chemical modifications etc, also relevant to the field of cancer treatment.
A general weakness with many drugs, including nucleoside analogues, is low activity and inferior specificity for treatment of the actual disease in question. Some of these problems may be related to the inherent activity of the drug substance itself, some may be related to certain resistance mechanisms (either inherent in the patient or acquired during treatment e.g. MDR in cancer treatment). Some problems may be related to certain inferior transport or cellular uptake and activation mechanisms. Some problems may be related to rapid inactivation and/or excretion of the drug.
The efficacy of nucleoside analogues depends on a large extent on their ability to mimic natural nucleosides, thus interacting with viral and/or cellular enzymes and interfering with or inhibiting critical processes in the metabolism of nucleic acids. In order to exert their antiviral or anti cancer activity, the nucleoside analogues have to be transformed, via their mono- and di-phosphates, into their corresponding tri-phosphates through the action of viral and/or cellular kinases. As a general rule, the tri-phosphate is the active agent, but for some products, e.g. gemcitabine, even the di-phosphate may exert a clinically significant effect.
In order to reach the diseased, cancerous or virus infected cells or tissues, following either enteral or parenteral administration, the nucleoside analogues should have favourable pharmacokinetic characteristics. In addition to rapid excretion of the administered drug, many nucleoside analogues may be deactivated both in the blood stream and in tissues. For instance may cytosine derivatives, even at the mono-phosphate level, be rapidly deaminated through the action of a class of enzymes called deaminases, to the inactive uracil analogue. The cellular uptake and thus good therapeutic efficacy of many nucleoside analogues strongly depend on membrane bound nucleoside transport proteins (called concentrative and equilibrative nucleoside transporters). Hence compounds that do not rely on such specific uptake mechanisms are sought for. Yet another activity limiting factor, particularly within the anti cancer field, are the cellular repair mechanisms. When an anti-cancer nucleoside analogue mono-phosphate is incorporated into the cellular DNA, it should not be removed from the cancer cell DNA due to the exonuclease activity linked to the p53 protein. However, removal of a nucleoside analogue from the DNA of a healthy cell is favourable in order to limit the side effects of the drug.
Over the years, many nucleoside analogues have been developed that to a large extent overcome some or many of the activity limiting features. As an example, acyclovir (ACV) can be given to illustrate a compound with great specificity. The ACV-mono-phosphate can only be formed by viral kinases meaning that ACV cannot be activated in uninfected cells. Despite this fact, ACV is not a particularly active product. In order to circumvent the often rate limiting step in the activation of a nucleoside analogue, the intracellular formation of the nucleoside analogue mono-phosphate, several phosphonate such as cidofovir or even mono-phosphate products, have been developed. In order to facilitate oral uptake or to secure a favourable drug disposition in the body, particular prodrugs such as Hepsera have been made.
In addition to the structural changes made to nucleoside analogues to facilitate enhanced clinical utility, further modifications have been made to improve the activity. The Applicant of the present invention (U.S. Pat. No. 6,153,594, U.S. Pat. No. 6,548,486 B1, U.S. Pat. No. 6,316,425 B1, U.S. Pat. No. 6,384,019 B1) and several other groups have modified nucleoside analogues through the addition of lipid moieties (EP-A-56265, EP-A-393920, WO 99/26958). This can be achieved by the linking of fatty acids through for instance an ester, amide, carbonate or carbamate bond. More elaborate products can be made, such as phospholipid derivatives (Eur J Pharm Sci (2000) 11b Suppl 2: p 15-27, EP 545966 B1, CA 2468099 A1, U.S. Pat. No. 6,372,725 B1 and U.S. Pat. No. 6,670,341 B1) of the nucleoside analogues. Such analogues are described to have antiviral activity that is particularly suitable for the therapy and prophylaxis of infections caused by DNA, RNA or retroviruses. They are also suited for treatment of malignant tumours. The nucleoside analogue lipid derivatives may serve several purposes. They may be regarded as a prodrug that is not a substrate for deaminases, thereby protecting the nucleoside analogues from deactivation during transport in the bloodstream. The lipid derivatives may also be more efficiently transported across the cellular membrane resulting in enhanced intracellular concentration of the nucleoside analogue. Lipid derivatives may also be more suited for use in dermal preparations, oral products (U.S. Pat. No. 6,576,636 B2 and WO 01/18013 or particular formulations such as liposomes (U.S. Pat. No. 5,223,263) designed for tumour targeting.
Previously, some of the inventors of the present invention have demonstrated that for nucleoside analogues with a conserved β-D configuration of the monosaccharide ring, or for nucleoside analogues with a non-cyclic side chain, the antiviral or anticancer activity can be most efficiently improved through the formation of lipid derivatives of mono-unsaturated ω-9 C18 and C20 fatty acids (Antimicrobial Agents and Chemotherapy, January 1999, p. 53-61, Cancer Research 59, 2944-2949, Jun. 15, 1999, Gene Therapy (1998) 5, 419-426, Antiviral Research 45 (2000) 157-167, Biochemical Pharmacology 67 (2004) 503-511). Not only being more active than the poly-unsaturated counterparts, the preferred mono-unsaturated derivatives are more crystalline and chemically stabile towards oxidation of the lipid chain, hence being more favourable compounds from a chemical and pharmaceutical manufacturing point of view. The Applicant of the present invention has also demonstrated that the mono-unsaturated ω-9 C18 and C20 fatty acids are suited for improvement of the therapeutic activity of a large number of non-nucleoside biologically active compounds (EP 0977725 B1).
A relatively new subgroup of nucleoside analogues are the so called 1,3-dioxolane derivatives. In this class of compounds, the five-membered ring, analogues to the monosaccharide found in natural nucleosides, is conserved, but the CH2 group in position 3 is exchanged with an O atom as shown in formula B below.

Several products within the 1,3-dioxolane class have shown promising antiviral and anti cancer activity in-vitro and in-vivo. For selected compounds, both enhanced, non-nucleoside transporter dependent, cellular uptake and reduced deamination (=inactivation) rate has been shown.
Several types of lipophilic derivatives, including both poly-unsaturated and mono-unsaturated ω-9 C18 and C20 fatty acid ester and amides, of such 1,3,-dioxolane products are known (U.S. Pat. No. 5,817,667, US 2001/0020026 A1 and US 2003/0013660 A1). These references provide both a general teaching about the possible advantages obtained by making such lipid derivatives and the specific benefit of the same regarding activity in disease and mechanistic models. In particular, the in-vitro anti cancer activity of the N4-amide derivatives of (−)-β-L-dioxolane-cytidine (troxacitabine) made with the following fatty acids, oleic acid (cis-9-octadecenoic acid), elaidic acid (trans-9-octadecenoic acid) and linoleic acid (cis-9,12-octadecadienoic acid), have been compared in several cell lines. The three products have quite comparable activity, the oleic acid amid derivative being marginally better (2 to 5 times) than the elaidic counterpart.